A major cause of colon removal surgery in veterans with inflammatory bowel disease (IBD) is a failure of medically-induced mucosal healing. Mucosal healing is a reliable clinical marker of recovery after IBD therapy as it is associated with durable clinical remission. Despite the importance of mucosal repair, we continue to have a poor understanding of many of the cellular and molecular elements that mediate ulcer healing. The current proposal focuses on the role of colonic (epithelial) stem cells (CSC) in ulcer healing during colitis. Wnt/?-catenin signaling in stem cells promotes cell proliferation as well as self-renewal, both essential functions needed for ulcer healing and restitution of the mucosal barrier. A central tenant of this proposal is the hypothesis that Wnt/?-catenin signaling plays an integral role in mucosal repair in IBD and that steroids delay ulcer healing my impairing Wnt/?-catenin signaling. In mucosal ulceration, as seen in IBD, there is replacement of surface mucosa with granulation tissue that is re-epithelialized with new crypt structures. We suspect that active Wnt signaling is required for several steps through this sequence of events from stage 1) generation of an intestinal epithelial cell (IEC) monolayer, to stage 2) formation of epithelial invaginations, to stage 3) formation of crypt islands on ulcer surfaces and finally stage 4) regeneration of mature crypts (Fig 3). Biochemical data from our lab indicate Wnt/?-catenin signaling is increased during ulcer healing in colitis in mice and IBD patients. In studies to interrogate the role of Wnt signaling in ulcer healing, we discovered that IEC expressing mRNA for the Wnt target gene Axin2 expand in ulcer margins, on ulcer surfaces, and within newly-formed crypt structures in the middle of ulcers. Studies using a novel Axin2 reporter mouse model indicate that Axin2+ IEC are pluripotent and capable of growing colonoid structures in vitro from a single cell. Together the preliminary data suggest that Axin2+ IEC represent a novel ISC population that forms new crypt structures during ulcer healing. In Aim 1, Axin2lacZ/+ and Axin2 CreERT2/+; R26RmTmG/+ mice will be used to study ulcer-associated Axin2+ IEC localization and gene expression profiles of this new stem cell population, as well as perform lineage tracing during ulcer healing in DSS colitis. In Aim 2 we will examine ulcer healing in mice treated with steroids. New studies will utilize Axin2-specific expression of stabilized ?-catenin to examine its role in ulcer healin during steroid therapy. Aim 3 studies will interrogate the role of Axin2+ IEC in human IBD and will analyze the mechanisms by which steroids impair ulcer healing. The goal of these studies will be to determine if untreated UC patients exhibit increased Axin2 expression, Wnt signaling and stem cell gene expression in areas of ulcer healing (Aim 3A) and determine whether steroid therapy reduces levels of Axin2 expression and stem cell activation in areas of delayed ulcer healing in a prospective trial of UC patients treated with steroids for a colitis flare (Aim 3B). Together the studies proposed will provide valuable insights into the molecular mechanisms that govern ulcer healing in colitis. The goal is to determine areas where therapeutic targets can be designed to safely accelerate ulcer healing and reverse the negative impact of steroids on mucosal repair in colitis.